cleanup: Port non-GObject classes to JS6 classes

ES6 finally adds standard class syntax to the language, so we can
replace our custom Lang.Class framework with the new syntax. Any
classes that inherit from GObject will need special treatment,
so limit the port to regular javascript classes for now.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/361

js/ui/extensionDownloader.js

@@ -1,7 +1,5 @@
 // -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*-
 
-const Lang = imports.lang;
-
 const Clutter = imports.gi.Clutter;
 const GLib = imports.gi.GLib;
 const Gio = imports.gi.Gio;
@@ -183,12 +181,10 @@ function checkForUpdates() {
     });
 }
 
-var InstallExtensionDialog = new Lang.Class({
-    Name: 'InstallExtensionDialog',
-    Extends: ModalDialog.ModalDialog,
-
-    _init(uuid, info, invocation) {
-        this.parent({ styleClass: 'extension-dialog' });
+var InstallExtensionDialog =
+class InstallExtensionDialog extends ModalDialog.ModalDialog {
+    constructor(uuid, info, invocation) {
+        super({ styleClass: 'extension-dialog' });
 
         this._uuid = uuid;
         this._info = info;
@@ -216,12 +212,12 @@ var InstallExtensionDialog = new Lang.Class({
         let label = new St.Label({ style_class: 'message-dialog-title headline',
                                    text: message });
         box.add(label);
-    },
+    }
 
     _onCancelButtonPressed(button, event) {
         this.close();
         this._invocation.return_value(GLib.Variant.new('(s)', ['cancelled']));
-    },
+    }
 
     _onInstallButtonPressed(button, event) {
         let params = { shell_version: Config.PACKAGE_VERSION };
@@ -264,7 +260,7 @@ var InstallExtensionDialog = new Lang.Class({
 
         this.close();
     }
-});
+};
 
 function init() {
     _httpSession = new Soup.SessionAsync({ ssl_use_system_ca_file: true });